This invention relates to the formation of substrates which contain a filler such that the filled substrate is rendered electrically conductive. In particular, this invention relates to such a process wherein the conductive filler is formed from discrete metal particles of copper or nickel.
It is frequently desired to incorporate a conductive metal filler in a (generally) non-conductive substrate. Such composites are useful for electromagnetic interference (EMI) shielding and so forth.
There is art that deals with the problem of forming a layer of conductive metal on the surface of a substrate. In contrast, the instant invention deals with the problem of rendering an entire three-dimensional substrate conductive.
U.S. Pat. No. 4,778,636 to Krieg et al. discloses a method of making plastic, electrically conductive plates by mixing an electrically conducting substance, such as carbon black or aluminum flake, with thermoplastic particles and pressure forming the mixture produced from the mixing step. Krieg et al. teaches the necessity of extreme high energy mixing of the plastic and conducting substance, i.e. by a "Retsch" centrifugal force ball mill. Thus, in Krieg et al. Example 10, where such mixing is employed, the use of aluminum flake in polycarbonate is successful, while in Example 11, where a simple vane stirrer is employed with the same ingredients, poor conductivity is obtained. It is also interesting to note the skin that develops on the Krieg et al. pressure-formed plates which must be removed by grinding or facing.
U.S. Pat. No. 3,708,387 to Turner et al. also discloses incorporating a conductive material, such as copper or nickel, in a plastic matrix. The reference warns that the metals should be used in as pure a form as possible and be free of surface oxidation. Turner et al. shows in its examples failure to obtain a conductive material due to an oxidized surface on the aluminum.
The present invention, in marked contradistinction to the above teachings, is able to make conductive a non-conductive powder blend of substrate and oxide coated metal particles without resorting to extreme means of mixing, such as with use of a ball mill.